1. Field of the Invention The subject invention relates to a hypodermic syringe having a sharp needle cannula and a rigid protective shield that can be locked in a position around the needle cannula for helping to prevent accidental needle sticks.
2. Description of the Prior Art Hypodermic syringes typically have a needle shield removably mounted over the needle cannula to avoid accidental needle sticks and to prevent damage to the needle before use. The needle shield can be safely removed and discarded when the hypodermic syringe is about to be used.
Accidental needle sticks occurring after the needle cannula has been used pose a substantially greater health risk, because the used needle cannula may be contaminated. Most health care facilities include sharps receptacles into which a used hypodermic syringe may be safely deposited. However, the hypodermic syringe is not always used near a sharps receptacle,and the needs of a patient may prevent the health care worker from traveling to the sharps receptacle immediately after using the hypodermic syringe.
The prior art includes hypodermic syringes with a safety shield telescoped over the syringe barrel and moveable between a proximal position where the needle cannula is exposed and a distal position where the needle cannula is surrounded. The prior art safety shield is retained in its proximal position until after the needle cannula is used. The safety shield then is moved distally on the syringe barrel to protectively enclose the needle cannula for preventing accidental needle sticks.
The prior art includes many structures for retaining the safety shield in its distal position after using the hypodermic syringe. For example, some prior art hypodermic syringes include an annular rib on the safety shield that can be engaged in an annular groove on the syringe barrel, as shown, for example, in U.S. Pat. No. 5,067,945.
The prior art also includes hypodermic syringes with bayonet-type interengagements between the safety shield and the syringe barrel which require both an axial movement and a twisting movement to retain the safety shield in either its proximal position or its distal position. A prior art hypodermic syringe with such a bayonet-type connection is shown in U.S. Pat. No. 5,024,660.
Other prior art hypodermic syringes rely upon a threaded interengagement between the safety shield and the syringe barrel, as shown in U.S. Pat. No. 4,743,233.
U.S. Pat. No. 5,059,185 teaches a safety shield with slots extending into the proximal end to define deflectable fingers which are configured to engage a continuous annular groove in an outer surface of the syringe barrel.
Other prior art hypodermic syringes form either the needle guard or the syringe barrel with a discontinuous wall that defines at least one deflectable finger intermediate the length of the needle guard. A locking detent at the end of the deflectable finger is engageable in an opposed groove. Hypodermic syringes of this general design are shown in U.S. Pat. No. 5,137,521 to Wilkens or U.S. Pat. No. 4,737,144 to Choksi. Additionally the prior art safety shields do not provide adequate locking of the needle guard in its distal position for protecting against re-exposure of the needle cannula. Furthermore, many of the prior art designs do not provide adequate tactile and audible indication of the safety shield being locked in its distal position for protectively surrounding the needle cannula.
Many of the prior art designs are very sensitive to dimensional tolerances, and hence are complex and costly to manufacture. Some prior art designs allow the safety shield to be extended to its most distal needle covering position while not being locked so that the device appears to be safe when it is not. Other prior art devices lock automatically upon extension, however, the user may not be sure that the needle guard is locked unless there is an audible, visual and/or tactile indication that such locking has occurred. Accordingly, although the prior art has addressed syringes having self-contained safety shields which can be locked to help prevent accidental needle sticks there is still a need for a simple, straight-forward, reliable, easily fabricated syringe with a locking safety shield having a locking structure which is less tolerance dependent while providing adequate locking force to hold the safety shield in the locked position.